The UN Secretary General Ban Ki Moon arrives in Sudan today for talks with President Omar al-Bashir, as aid agencies and diplomats warn that the situation in Darfur is deteriorating.

As well as trying to smooth the way for a speedy deployment of a joint UN/African Union force of 26,000 personnel, Mr Ban is planning to press the Sudanese leader to sit down again with the Darfur rebels, now that the numerous factions have agreed a common negotiating position.

"My goal is to lock in the progress we have made so far, to build on it so that this terrible trauma may one day cease," Mr Ban said.

Critics note that internal divisions still plague the Darfur rebel movements, making a lasting peace deal remote, and they are also worried that Khartoum may play for time over the deployment of peace keepers, as it has in the past.

On the humanitarian side, the outlook is no rosier. The UN chief's visit comes just after one of his senior humanitarian officials, Margareta Wahlstrom, warned that in many parts of Darfur, the humanitarian situation was deteriorating. Around a quarter of a million people have fled in the last eight months. Khartoum and the rebels accused each other of launching attacks in recent weeks that have killed civilians.

The increasing violence and the state of lawlessness is also having a devastating effect on malnutrition rates. A series of 18 surveys by the UN and aid agencies has found the number of malnutrition cases has increased beyond the emergency threshold of 15 percent of the population to the highest level in three years.

UN officials in Darfur are also reporting increases in attacks on aid workers. The number of incidents, including carjackings, hijacking of convoys and shootings, has risen by 150 per cent. Many roads outside Darfur's three state capitals are now deemed too dangerous to travel on, meaning large swathes of the region, which is twice the size of the UK, is now considered off-limits for aid workers.

Before the conflict started in 2003, Darfur was home to around seven million people, mainly from three African tribes – the Fur, Marsalit and Zargahwa. But some 2.5 million have fled their homes after attacks by Sudanese troops and planes, and Arab militia on horseback known as the Janjaweed. Most of the displaced are now living in camps around Darfur's main towns, while around 250,000 are in Chad. It is estimated that more than 200,000 people have been killed so far during the four and a half year conflict.

Despite Khartoum's grudging acceptance of a joint UN/AU force, the Western powers on the UN Security Council are still threatening sanctions. Two senior Western diplomats and the director of an aid agency were expelled from Sudan last week.

Prime Minister Gordon Brown and French President Nicholas Sarkozy last week said the threat was still necessary. In a joint statement, they said they would work for "further sanctions against those who fail to fulfil their commitments, obstruct the political process or continue to violate the ceasefire".

But Sudan's closest ally, China, last night reiterated its opposition to sanctions. China's ambassador to Sudan, Li Chengwen, told Reuters: "Dialogue is better than pressure because with sanctions, who will suffer in the end – the people."